Despite new technologies that significantly aid in reducing exhaust emissions from internal combustion diesel engines, NOx and particulate matter (PM) emissions are still a subject of environmental concern. The term “NOx” includes the mono-nitrogen oxides NO and NO2 (nitric oxide and nitrogen dioxide). Emission standards for vehicle engines are becoming increasingly stringent, and it is difficult to meet governmental emissions regulations by merely relying only on improvements to the engine itself. Thus, today's engines continue to require some sort of exhaust gas aftertreatment system.
Modern aftertreatment catalysts such as three-way catalysts for gasoline engines and diesel oxidation catalysts for diesel engines effectively oxidize unburned hydrocarbon emissions with very high efficiencies once the catalysts reach a minimum temperature. However, when an engine is started after a prolonged period of inactivity, the catalysts are cold and ineffective until they warm up. Emissions emitted during the warm-up period constitute the majority of emissions emitted from the tailpipe. Control of these “cold-start” emissions is critical for meeting current and future emissions standards.